In an effort to meet stringent federal government emissions standards, engine systems may be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. Additionally various sensors may be included in the engine exhaust manifold to estimate tailpipe emissions.
One example of such an EGR system is illustrated by Lutz et al. in US 2008/0223038 A1. Herein, a low pressure EGR (LP-EGR) system is configured to recirculate exhaust gas from downstream of a particulate filter to the engine intake. An amount of EGR is adjusted via an LP-EGR valve positioned in the LP-EGR bypass. Additional EGR adjustments may be performed via an exhaust gas back-pressure valve positioned downstream of the EGR bypass. Further, various sensors (for example, a NOx sensor, an air-fuel ratio sensor, and a particulate matter sensor) may be included in the engine exhaust manifold to estimate tailpipe emissions.
However, the inventors herein have recognized a potential issue with such systems. Exhaust gas constituents may not be uniformly distributed. Consequently, there may be a discrepancy between the concentration of an exhaust gas constituent as estimated by a sensor in the exhaust manifold, and the concentration of the constituent in the bulk exhaust gas. Thus, the accuracy of the sensor may be degraded leading to degraded engine emissions. While EGR valve adjustments may provide some exhaust gas mixing, even small EGR valve modulations may have large effects on an amount of EGR provided, thereby degrading the desired EGR control. Fixed exhaust mixers may alternatively be included in the engine exhaust to improve exhaust gas mixing, however, such mixers may add substantial component costs.
Thus, in one example, the above mentioned issue may be addressed by improving exhaust gas mixing through a method of operating an engine including an EGR system coupled between an engine intake and an engine exhaust. The method may comprise, adjusting a back-pressure valve of the EGR system to provide a desired EGR amount, the adjusting below a first frequency, and selectively modulating the back-pressure valve above the first frequency to promote exhaust gas mixing, while maintaining the desired EGR amount.
In one example, an engine may include an exhaust gas recirculation (EGR) loop coupling the engine exhaust manifold to the intake manifold. The low pressure EGR loop may recirculate at least some exhaust gas from downstream of an emission control device in the exhaust manifold, to upstream of an intake throttle in the intake manifold. An amount of exhaust gas recirculated to the intake may be adjusted by ways of an EGR valve included in the EGR loop. An additional back-pressure valve, positioned in the exhaust manifold, downstream of the EGR loop and the emission control device, and upstream of exhaust emission sensors, may be configured to provide additional back-pressure to drive exhaust gas flow into the EGR loop. During selected engine operating conditions, EGR may be provided by adjusting the EGR valve and the back-pressure valve. For example, an amount of EGR may be increased by opening the EGR valve, and partially closing the back-pressure valve. The back-pressure valve may be adjusted at a lower frequency (or below a predefined frequency) to provide the desired amount of EGR. Herein, by recirculating heated exhaust gas to the engine intake, engine exhaust emissions and fuel economy may be improved.
In one example, the emission control device may include a diesel particulate filter (DPF). Herein, to improve filter operations, and consequently exhaust emissions, exhaust NOx and/or particulate matter (PM) levels may be periodically sensed by one or more exhaust sensors positioned downstream of the DPF and the back-pressure valve. During selected conditions when sensing is requested, (for example, when sensing is requested to determine whether the filter needs to be regenerated, or when sensing is requested for filter leak detection routines), the back-pressure valve may be modulated (for example, sinusoidally pulsed) to create a local turbulence that mixes the exhaust gas. Specifically, the back-pressure valve may be selectively modulated at a higher frequency (or above the predefined frequency) such that exhaust gas mixing is achieved without affecting the average EGR flow. For example, the back-pressure valve may be adjusted with a first frequency during a first EGR condition when no sensing is requested. Then, during a second EGR condition, when sensing is requested, in addition to the adjustment, the back-pressure valve may be modulated with a second, higher frequency component.
The modulation frequency, during sensing, may be determined based on engine operating conditions, and may be selected so as to maintain the requested amount of EGR. For example, the frequency may be selected such that the average back-pressure valve position is maintained at the position desired for the requested amount of EGR. In one example, determining the modulation frequency based on engine operating conditions may include increasing the frequency of modulation as an engine speed increases. Additionally, the amplitude of modulation may be adjusted based on engine operating conditions. For example, the adjustment may include decreasing the amplitude of the modulation as engine valve timing approaches valve opening or valve closing limits.
In still another embodiment, the EGR valve may be selectively modulated in addition to the back-pressure valve modulation during sensing. The EGR valve may be an HP-EGR valve and/or an LP-EGR valve. Herein, the EGR valve modulation may be adjusted in coordination with the back-pressure valve modulation such that the requested amount of EGR is maintained. For example, the frequency (and amplitude) of EGR valve modulation may be adjusted to match the frequency of the back-pressure valve modulation, while a phase of the EGR valve modulation may be adjusted to off-set the back-pressure valve modulation.
In this way, by applying a periodic motion to the back-pressure valve, a local stirring of exhaust gases may be achieved that may improve exhaust gas mixing without affecting EGR flow. By improving the mixing of exhaust gases, the sensing accuracy of exhaust gas sensors may be improved. Additionally, by using components already in use in EGR systems to actively mix the exhaust gas, the need for dedicated mixers, such as fixed exhaust gas mixers, may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.